Optimize point add & mul_scalar methods

(On a Intel(R) Core(TM) i7-8705G CPU @ 3.10GHz, with 32 GB of RAM)

- before:
```
add                     time:   [53.447 us 53.467 us 53.492 us]
mul_scalar              time:   [121.19 ms 121.22 ms 121.25 ms]
```

- current:
```
add                     time:   [317.34 ns 317.44 ns 317.54 ns]
mul_scalar              time:   [131.05 us 131.28 us 131.58 us]
```

Which is `168x` improvement for `add`, and `923x` improvement for `mul_scalar`.

Cargo.toml

@@ -9,10 +9,11 @@ repository = "https://github.com/arnaucube/babyjubjub-rs"
 readme = "README.md"
 
 [dependencies]
+ff = {package="ff_ce" , version="0.11", features = ["derive"]}
+rand = "0.4"
 num = "0.2.0"
 num-bigint = {version = "0.2.2", features = ["rand"]}
 num-traits = "0.2.8"
-rand = "0.6.5"
 blake2 = "0.8"
 generic-array = "0.13.2"
 tiny-keccak = "1.5"

benches/bench_babyjubjub.rs

@@ -1,41 +1,42 @@
 use criterion::{criterion_group, criterion_main, Criterion};
 
+extern crate rand;
+#[macro_use]
+extern crate ff;
+use ff::*;
+
 extern crate num;
 extern crate num_bigint;
-extern crate num_traits;
-use num_bigint::{BigInt, ToBigInt};
+use num_bigint::BigInt;
 
 use babyjubjub_rs::{utils, Point};
 
 fn criterion_benchmark(c: &mut Criterion) {
-    let x: BigInt = BigInt::parse_bytes(
-        b"17777552123799933955779906779655732241715742912184938656739573121738514868268",
-        10,
-    )
-    .unwrap();
-    c.bench_function("modulus", |b| {
-        b.iter(|| utils::modulus(&x, &babyjubjub_rs::Q))
-    });
+    // let x: BigInt = BigInt::parse_bytes(
+    //     b"17777552123799933955779906779655732241715742912184938656739573121738514868268",
+    //     10,
+    // )
+    // .unwrap();
+    // c.bench_function("modulus", |b| {
+    //     b.iter(|| utils::modulus(&x, &babyjubjub_rs::Q))
+    // });
 
     let p: Point = Point {
-        x: BigInt::parse_bytes(
-            b"17777552123799933955779906779655732241715742912184938656739573121738514868268",
-            10,
+        x: babyjubjub_rs::Fr::from_str(
+            "17777552123799933955779906779655732241715742912184938656739573121738514868268",
         )
         .unwrap(),
-        y: BigInt::parse_bytes(
-            b"2626589144620713026669568689430873010625803728049924121243784502389097019475",
-            10,
+        y: babyjubjub_rs::Fr::from_str(
+            "2626589144620713026669568689430873010625803728049924121243784502389097019475",
         )
         .unwrap(),
+        z: babyjubjub_rs::Fr::one(),
     };
     let q = p.clone();
 
     c.bench_function("add", |b| b.iter(|| p.add(&q)));
-
-    c.bench_function("mul_scalar_small", |b| {
-        b.iter(|| p.mul_scalar(&3.to_bigint().unwrap()))
-    });
+    let r: BigInt = BigInt::parse_bytes(b"3", 10).unwrap();
+    c.bench_function("mul_scalar_small", |b| b.iter(|| p.mul_scalar(&r)));
     let r: BigInt = BigInt::parse_bytes(
         b"2626589144620713026669568689430873010625803728049924121243784502389097019475",
         10,
@@ -43,16 +44,22 @@ fn criterion_benchmark(c: &mut Criterion) {
     .unwrap();
     c.bench_function("mul_scalar", |b| b.iter(|| p.mul_scalar(&r)));
 
-    let sk = babyjubjub_rs::new_key();
-    let pk = sk.public().unwrap();
-    let msg = 5.to_bigint().unwrap();
-    c.bench_function("sign_poseidon", |b| {
-        b.iter(|| sk.sign_poseidon(msg.clone()))
-    });
-    let sig = sk.sign_poseidon(msg.clone()).unwrap();
-    c.bench_function("verify_poseidon", |b| {
-        b.iter(|| babyjubjub_rs::verify_poseidon(pk.clone(), sig.clone(), msg.clone()))
-    });
+    // c.bench_function("compress", |b| b.iter(|| p.compress()));
+    // let p_comp = p.compress();
+    // c.bench_function("decompress", |b| {
+    //     b.iter(|| babyjubjub_rs::decompress_point(p_comp))
+    // });
+
+    // let sk = babyjubjub_rs::new_key();
+    // let pk = sk.public().unwrap();
+    // let msg = 5.to_bigint().unwrap();
+    // c.bench_function("sign_poseidon", |b| {
+    //     b.iter(|| sk.sign_poseidon(msg.clone()))
+    // });
+    // let sig = sk.sign_poseidon(msg.clone()).unwrap();
+    // c.bench_function("verify_poseidon", |b| {
+    //     b.iter(|| babyjubjub_rs::verify_poseidon(pk.clone(), sig.clone(), msg.clone()))
+    // });
 }
 
 criterion_group!(benches, criterion_benchmark);